Root responses to cadmium in the rhizosphere: a review

Abstract

This article reviews the responses of plant roots to elevated rhizosphere cadmium (Cd) concentrations. Cadmium enters plants from the soil solution. It traverses the root through symplasmic or apoplasmic pathways before entering the xylem and being translocated to the shoot. Leaf Cd concentrations in excess of 5-10 μg g(-1) dry matter are toxic to most plants, and plants have evolved mechanisms to limit Cd translocation to the shoot. Cadmium movement through the root symplasm is thought to be restricted by the production of phytochelatins and the sequestration of Cd-chelates in vacuoles. Apoplasmic movement of Cd to the xylem can be restricted by the development of the exodermis, endodermis, and other extracellular barriers. Increasing rhizosphere Cd concentrations increase Cd accumulation in the plant, especially in the root. The presence of Cd in the rhizosphere inhibits root elongation and influences root anatomy. Cadmium concentrations are greater in the root apoplasm than in the root symplasm, and tissue Cd concentrations decrease from peripheral to inner root tissues. This article reviews current knowledge of the proteins involved in the transport of Cd across root cell membranes and its detoxification through sequestration in root vacuoles. It describes the development of apoplastic barriers to Cd movement to the xylem and highlights recent experiments indicating that their maturation is accelerated by high Cd concentrations in their immediate locality. It concludes that accelerated maturation of the endodermis in response to local Cd availability is of functional significance in protecting the shoot from excessive Cd loads.

Pb and Cd on growth, leaf ultrastructure and essential oil yield mint (Mentha arvensis L.)

“…Similarly, the Cd also showed much higher concentrations in roots (78mg kg -1 ) than in the leaves (12mg kg -1 ) of M. arvensis, possibly due the restriction of Cd transport by xylem, which may occur by increased lignin deposition on these cells (LUX et al, 2011). Premature lignification of vessel element adds layers of impermeable cells that reduce transport of both water and metal (LUX et al, 2011).…”

“…Premature lignification of vessel element adds layers of impermeable cells that reduce transport of both water and metal (LUX et al, 2011). However, the high Cd concentration in M. arvensis leaves, forbid its popular use as decoction or infusion, once the plant processing in hot water can extract the metals for the tea (ABOU-ARAB & ABOU DONIA, 2000).…”

Differential cadmium stress tolerance in five Indian mustard (Brassica junceaL.) cultivars

“…3,4 Although Cd is a highly phytotoxic metal, it is easily taken up by plant roots growing on Cd-contaminated soils and transported to above ground plant parts. [5][6][7] The regulatory limit of Cd in agricultural soils is 100 mg Cd kg -1 soil, 8 but this threshold is continuously increasing because of anthropogenic and industrial activities. Plants growing on Cd contaminated soil result in Cd accumulation in all plant parts, which inhibits plant growth, affect nutrient uptake, alters the chloroplast ultrastructure, inactivates enzymes of CO 2 fixation, inhibits photosynthesis and induces lipid peroxidation and antioxidant machinery.…”

“…8,10,19,23,33 Higher Cd accumulation in roots than leaves suggest that Cd transport to the xylem is restricted, therefore less in above ground parts, suggesting that Cd is not readily translocated in the phloem. 6,34,35 Furthermore, Cd accumulation in the root and leaf also depends on binding to the extracellular matrix. 36 It has also been reported that Cd accumulation by plants grown in soil is directly related to transpiration and thereby to stomatal conductance.…”

“…19,37 Further, the ability of plants to grow on Cd contaminated soils is generally correlated with the ability of roots to exclude Cd from the plant and/or of plant tissues to chelate Cd as a non-toxic compound or sequester it in a non-vital cellular compartment. 6,38 Exposure to heavy metals results in the excessive production of ROS in plants. Unlike Cu and Fe, Cd generates ROS by catalyzing Haber-Weiss or Fenton type reactions.…”